Rapid imaging sequences or steady-state free precession (abbreviated, “SSFP”) sequences acquire imaging data in a dynamic equilibrium state (i.e., the so called “steady-state”). Due to the heavy duty requirements on system performance, these types of SSFP sequences were not in use until recently. Presently, such imaging sequences have become increasingly popular due to their very fast scanning properties, which are necessary for imaging fast dynamic processes, in abdominal imaging, in cardiac imaging, or when imaging very ill and traumatized patients. There are many different types of steady-state imaging sequences, and a special member of this group is obtained if the time integral of each of the three gradients is zero—e.g. “balanced”—within each repetition time (TR) (see FIG. 1). Such a pulse sequence is disclosed, for example, as FIG. 3 in U.S. Pat. No. 4,769,603, and was originally referred to by the acronym “FISP.” However, FISP has been more recently renamed “TrueFISP” (Siemens), or balanced steady-state free precession (b-SSFP) imaging. The excellent signal-to-noise ratio compared to other SSFP sequences and the T2-like contrast makes it useful for several clinical applications.
Although TrueFISP sequences are, to some point, quite robust against long-term and smooth changes of imperfections disturbing the perfectly balanced gradient scheme, rapid and discontinuous changes of residual spin dephasing induced by such imperfections may generate significant signal instabilities and fluctuations leading to image artefacts. One source of image artefacts is the rapidly changing eddy-currents generated during the excitation train and produced by the changing phase encoding (PE) gradients.
It is known that, except for the linear k-space trajectory with small variations between consecutive encoding steps, other encoding schemes such as centric, random or segmented orderings exhibit significant jumps and consequently induce rapidly changing eddy-currents. Another significant source of imperfections in the balanced gradient scheme are motional or flow related imperfections. Although the phase of static spins are perfectly balanced prior to the next excitation, a moving spin picks up a residual phase as a consequence of the motion-sensitive PE gradients. Strictly speaking, TrueFISP sequences are balanced with regard to static spins, (e.g. zeroth order gradient moments are balanced or “nulled” within each repetition time of the excitation train); however, the TrueFISP scheme is not completely balanced with regard to motion or flow where, in addition to the zeroth order moments, there are also first order moments that have to be compensated for.
In summary, phase encoding is the source of two kind of perturbations in the completely balanced TrueFISP gradient scheme, namely: (1) Eddy-current related residual spin-dephasing, and (2) Flow or motion related residual spin-dephasing. With the exception of linear view ordering, also referred to as the “standard” view ordering, where the above two kinds of perturbations are generally smooth and sufficiently small so as not to induce significant signal instabilities in the steady-state, imaging using other non-standard view-orderings, such as centric or segmented view orderings, is significantly affected by eddy currents and/or flow or motion artefacts due to large changes in the amplitude of the phase encoding gradients.
Independently of the origin of these sources of imperfections in the balanced gradient scheme, e.g., eddy-currents or flow, the resulting disturbances induce, dependent on the view ordering used, significant image artefacts, and thus compensation strategies are essential to removing these artefacts from the final image. Direct annihilation of the eddy-current related spin-phase perturbations by applying additional, opposing magnetic fields has been previously investigated, but was limited by uncertainty of the time evolution of induced eddy-currents and other higher order effects. Therefore, there remains a need for a generic compensation strategy that is system and sequence independent and that compensates for eddy-current related residual spin-dephasing, and flow or motion related residual spin-dephasing.
It is generally known that during MR imaging using TrueFISP sequences, pulsed gradients induce eddy-currents in the conducting parts of the magnet, which in turn generate unwanted additional varying magnetic fields that lead to imperfections in the applied gradient scheme. Typically, these induced time-varying fields decay multi-exponentially with time constants on the order of tens to hundreds of milliseconds. Mechanical compensation mechanisms, such as pre-emphasis circuits and shielded coils, are built into state-of-the-art MRI systems to eliminate many of these additional artefactual fields. However, it is generally not possible to perfectly compensate for all of the eddy current-related effects, and consequently, some residual induced fields still persist. Unfortunately, TrueFISP sequences are highly prone to generating imaging imperfections due to these residual fields because TrueFISP sequences require perfectly balanced gradient schemes.
Nevertheless, TrueFISP sequence imaging has the advantage of a dynamic equilibrium state that is quite stable against: (i) Constant residual imperfections, e.g. field inhomogeneities; and (ii) a repetitively constant gradient scheme, as applied in read and slice directions (in 2D acquisitions), which will essentially induce a constant eddy current-related spin dephasing, and establish a corresponding stable steady state.
Consequently, the above mentioned imperfections (i.e., residual induced fields) can be refocused by the steady-state and only a permanently changing gradient scheme, as used in the phase encoding direction, may induce sufficiently rapid changes in eddy-currents to produce significant signal instabilities and image artefacts. The resulting fluctuations are thus very closely related to the applied view ordering of the PE gradients, as demonstrated in FIG. 2. FIG. 2 demonstrates that image artefact is different for different view orders. A large change in k-space position, or PE gradient amplitude, between adjacent repetitions (i.e., consecutive phase encoding steps) applied close to the center of k-space is especially critical for the balanced SSFP scheme, as shown by the examples in FIG. 2 corresponding to linear, centric, or random view orderings. Repetitively applied linear PE schemes show minor artefacts since signal instabilities are confined to the outer parts of k-space. As a practical matter, TrueFISP sequences have been limited to application to MR imaging using linear view-orderings only.
In other words, the prior art TrueFISP imaging has been limited by the following technical problem. Balanced SSFP is especially sensitive to residual imperfections (i.e., eddy-current related residual magnetic fields & motion or flow related uncompensated residual first order phase encoding moments) that are induced by the stepwise changing of phase encoding gradients. With the exception of view orderings employing linear k-space trajectories with small variations between consecutive phase encoding steps, any other encoding scheme, such as centric, random or segmented view orderings, exhibit significant jumps between adjacent k-space positions. These relatively large jumps between adjacent k-space positions consequently induce rapidly changing eddy-currents. As demonstrated in FIG. 2, the resulting image artefacts depend on the chosen PE scheme. In contrast to the linear view ordering (also known as the standard view ordering) shown in column A, upper row, of FIG. 2, other alternative view-orderings induce significant perturbations in steady-state magnetization, and thus yield image artefacts and image degradations. Such a similar behaviour is not observed in non-balanced SSFP sequences, such as shown for a GRASS (non-balanced read and slice gradient, balanced phase encode gradient, no rf spoiling) sequence provided in column D in FIG. 2.
Thus, phase encoding gradients are a source of two kinds of imperfections: (i) Eddy-current related imperfections as shown in FIG. 2, and (ii) motion or flow related imperfections as shown in FIG. 3. The eddy current related imperfections are the result of abrupt changes in the characteristics (amplitude, rise times, timing . . . ) of image encoding gradients, as generated by non-linear view orderings, which induce discontinuous changes in eddy-current related residual magnetic fields. The flow or motion related imperfections are the result of abrupt changes in the residual first order moment of image encoding gradients, as generated by non-linear view orderings, which induce discontinuous changes in residual spin-phase. These abrupt changes lead to imperfections (e.g. an additional spin phase) in the balanced TrueFISP sequence scheme that cannot be refocused by the steady-state and thus leads to image degradations and artefacts.
Thus rapid and abrupt changes in the PE gradients, as a consequence of alternative view-orderings, such as centric, random, segmented and others, may induce significant perturbations in the steady-state leading to image artefacts and degradations. Methodological compensation mechanisms to correct for these imaging related imperfections in the totally balanced gradient scheme of TrueFISP sequences, in order to allow application of arbitrary view-orderings, are needed. There has heretofore been no compensation method for compensating for phase encoding related perturbations in steady-state magnetization of balanced SSFP imaging that allows arbitrary, e.g. non-linear, view orderings in TrueFISP sequences.
Thus, the present invention endeavors to provide a method of steady-state free precession MR imaging that maintains the advantages of balanced SSFP imaging while overcoming the limitations of the prior art methods.
Accordingly, one object of the present invention is to overcome the disadvantages and limitations of the prior art balanced SSFP methods.
Another object of the present invention is to provide a method of steady-state free precession MR imaging that compensates for phase encoding related perturbations in steady-state magnetization of balanced SSFP imaging so as to permit arbitrary, e.g. non-linear, view orderings in TrueFISP sequences.